During cardiopulmonary bypass surgery (CPB), the most common operation conducted in North America, the heart is stopped and the blood which normally returns to the right side of the heart passes through a pump and oxygenating system and is returned to the aorta, thereby bypassing the heart and lungs. The flow of blood is essentially non-pulsatile with a low amplitude waveform having monotonous regularity.
Although a common procedure (in excess of 400,000 open heart procedures per annum are conducted in North America) and although tremendous strides have been made so that open heart surgery is safer for patients, the procedure is not without its dangers. Although the vast majority of patients have marked improvement in their cardiac functional status following their procedure, of concern is the potential for damage to other organ systems which can occur due to the need for CPB.
The following consequences have been identified with conventional non-pulsatile CPB, namely metabolic acidosis, interstitial fluid accumulation, elevated systemic vascular resistance, arteriovenous shunting and impaired brain oxygenation. Of greatest concern is the potential for neurologic damage. Increasingly, well conducted prospective trials have demonstrated an alarming rate of post-operative neuropsychologic disturbances following cardiac surgery. Recent studies have shown that up to 60 percent of patients undergoing open heart surgery have neuropsychologic deficits following their operation, so that as many as 240,000 patients per annum may develop neurologic abnormalities following cardiac surgery. These disturbances are subtle but involve higher cognitive functions of the brain.
Mechanical ventilation of the lungs represents one of the major accomplishments of modern medicine and is one of the cornerstones upon which modern surgery and intensive care is based. Despite many major advances, mechanical ventilation is still associated with a number of alterations in respiratory function which causes morbidity and mortality in patients requiring this type of support. Inability to maintain gas exchange remains one of the major limiting factors with regard to life-support of critically ill patients. Even in healthy patients being ventilated during elective surgery, alterations in gas exchange can be demonstrated. These relate to collapse of small airways and alveoli. Prevention of these alterations would likely represent a major advance in management of all patients requiring ventilatory support. Conventional mechanical ventilation is monotonously regular in delivery of set tidal volume and respiratory rate.
The monotonous regularity of pumping of blood during CPB and of set tidal volume and respiratory rate of a mechanical ventilator is in contrast to the intrinsic spontaneously variable rhythms of heart rate, blood pressure and respiration, associated with a normal functioning heart as well as the considerable range of tidal volume and respiratory rate which a healthy individual demonstrates during breathing.